Scenario

Transmission utility in the Middle East with a fleet of strategic transformers (380 kV, up to 500 MVA), operating under high load and extreme climatic conditions.
A diagnostic campaign highlighted the widespread presence of DBDS and corrosive sulfur compounds in the insulating oils, with potential risk of metallic sulfide formation and consequent failures.

Technical characteristics

• 22 units (transformers, autotransformers, reactors)
• Power rating: 60–500 MVA
• Voltage: 380 kV
• Total oil: approx. 1,710,000 kg; up to 130,000 kg per unit

Problem

Analyses revealed initial concentrations of DBDS between 135 and 171 mg/kg and TCS up to 189 mg/kg.
According to CIGRE guidelines, such values combined with high temperatures and heavy load result in a high risk of corrosion and dielectric degradation.
The utility required an effective long-term solution compatible with service continuity.

Solution

In cooperation with GCC Lab, the utility selected the Sea Marconi selective depolarization, considered the most effective technique for removing DBDS and sulfur compounds, avoiding the limitations of traditional clay-based treatments.

The intervention was performed on-load, using Decontamination Modular Units (DMU) operating 24/7, featuring:

• closed-loop circulation of the entire oil volume;
• chemical–physical treatment using solid reagent and absorbent materials;
• integration of filtration, degassing, and dehydration;
• dedicated connections to the various compartments (main tank, OLTC, conservator).

Outage windows were limited to a few hours for connection/disconnection.

Results

In the first 12 months, 8 transformers were treated, totalling 809 tonnes of oil.

Results after treatment:

• DBDS < 5 mg/kg (below detection limit and well below the required < 10 mg/kg)
• TCS < 2.5 mg/kg
• Chemical–physical properties (water, TAN, DF, IFT, breakdown voltage) fully compliant with IEC 60422 ed. 4; many values comparable to new oils
• No significant alarms during treatment
• Effectiveness confirmed after 1 year: oils non-corrosive on copper and silver, DBDS stable < 5 mg/kg, parameters within IEC limits

The treatment allowed the utility to keep critical units in service, reducing operational risks and improving long-term oil quality.

Primary alluminiun factory: 8 Rectifier Transformers (Sweden)

Nameplate data of transformers:

Manufact: xxx
Year: 1930
Power [MVA]: 19
Voltage [KV]: 13
Oil Mass [Kg]: 15.000
Oil type: mineral blended

Selective depolarization treatment (Chedcos) in order to remove the DBDS + dehalogenation treatment (CDP Process) in orderr to remove the PCBs both from oil and transformers. The treatments were performed on-site, on-line and on-load (transformer in servise and under charge during the treatment).

The treatments were carried out with the following guarantees: DBDS < 10 mg/kg, PCBs < 2 mg/Kg restoring also the other oil parameters as suggested by IEC 60422 ed. 4, 2013

Company involved in production of primary aluminum with 8 rectifiers transformers

Nameplate data of the transformers:

Transformer: rectifier, ODAF with membrane
Manufact: xxx
Year: 1998
Power [MVA]: 93,4
Voltage [KV]: 66
Oil mass [Kg]: 27.200
Oil type: naphthenic

First failure with discharges of high energy (D2)

Post failure analysis, oil fingerprint with identification ant quantification of the copper sulphide on enameled contacts

Sea Marconi expertise recommended the selective depolarization treatment (Chedcos)

Company in the chemical business, 4 GSU transformer

Nameplate data of the transformer:

Manufatc: xxx
Year: 2000
Power [MVA]: 47
Voltage [KV]: 34
Oil mass [Kg]: 12.000
Oil type: naftenico

Primary substation with 3 Grid transformer and 2 shunt reactors

Nameplate data of the transformer:

Manufact: xxx
Year: 1998
Power [MVA]: 250
Voltage [KV]: 345
Oil mass [Kg]: 118.000
Oil type: naphthenic

The analysis and subsequent diagnosis performed by Sea Marconi on the machines of the customer showed a clear presence of DBDS

4 GSU transformer at thermal power plant

Nameplate data of the transformers:

Manufact: xxx
Year: 1997
Power [MVA]: 192
Voltage [KV]: 400
Oil mass [Kg]: 58.000
Oil type: naphthenic

Failure caused by discharges of high energy

Internal inspection and post failure analysis performed by Sea Marconi experts

Company involved in energy transmission (36 transformers)

Nameplate data of transformers:

Power max [MVA]: 528
Voltage max [KV]: 400
Oil mass [Kg]: 96.900 each
DBDS max [mg/Kg]: 185
DBDS target [mg/Kg]: <5

Brazilian energy transmission network of TSN e Novatrans with 76 electrical equipment filled with insulating fluids.

Discovery of DBDS. Sea Marconi, first in the world, in July 2005 identifies the DBDS as the main responsible of the faults attributed to the “corrosive sulfur”.

The solution was a campaign of selective depolarization treatments (Chedcos) by Sea Marconi. The treatments were performed on-site both off-load and on-load (transformer fully load during the treatment).
The Brazilian energy transmission network chose all the countermeasures available:

• replacing the corrosive oil with a new one not corrosive
• passivation with Irgamet 39
• selective depolarization treatment (Chedcos)

Chedcos® offers the advantage of preventing the operational criticalities correlated with the change of the oil that would be, in any case, contaminated by the DBDS present in the residual oil impregnating the papers. From the failure case history reported also by CIGRE 378 it results also that the passivator (e.g. Irgamet 39) does not eliminate the cause of the corrosion (the DBDS), it degrades in a few months and its protective action on the copper surfaces results being inhomogeneous and ineffective.